The use of flame retardant additives to polymers, such as polyolefins, polyesters, polycarbonates, polyamides, polyurethanes, epoxy resins, and other thermoplastic or thermoset polymer resins, is long known. It is also well known that the stability of the resultant flame retardant/polymer composition, for example short or long term thermal stability, stability on exposure to other environmental agents or other chemical components, etc., can be compromised depending on the resins and particular flame retardants employed. The thermal processing of polymers, e.g., thermoplastic, elastomeric or thermoset polymers often exposes a polymer composition to high temperatures under severe conditions.
For example, some polymers are processed at high temperatures, for example 200° C., 220° C., 250° C. or higher, and many known flame retardants are not suitable for use under these conditions because they are too volatile, not sufficiently thermally stable, have an adverse effect on the chemical or mechanical properties of the resin, etc. Certain organophosphorus flame retardant compounds, such as some phosphate esters, can also exhibit a plasticizing effect which may adversely affect mechanical properties of the polymers into which they are added. Some compounds are not stable in certain polymers under processing conditions, for example, some phosphates are relatively unstable to hydrolysis, which can not only deplete the flame retardant but can result in undesired formation of various phosphoric acid compounds and degradation of the resin.
Salts of phosphorus containing acids are known flame-retardant additives, for example, U.S. Pat. No. 3,894,986 discloses flame retardant thermoplastic polyesters containing alkali salts of phosphoric acids; U.S. Pat. No. 4,972,011 discloses aluminum salts of alkylphosphonic acids or mono-alkyl esters of alkane-phosphonic acids, i.e., salts of compounds of formula (Ia), wherein R is for example methyl, ethyl, propyl or isopropyl etc., and R′ is hydrogen, methyl, ethyl, propyl, or isopropyl.

DE 3833977 discloses the preparation of metal salts of compounds of formula (Ia) via reactions of dimethylmethylphosphinate and metal oxides or hydroxides in water at high pressures and temperatures from 120 to 200° C. and adducts of these salts with amines such as ethylene diamine and melamine, and use of the adducts as flame retardants in thermoplastics.
Salts of phosphinic acids, i.e., compounds of formula (II) wherein R1 and R2 are alkyl or carbon based aromatic, are also known flame-retardant additives for thermoplastic polymers.

Salts wherein M is selected from Mg, Ca, Al, Sb, Sn, Ge, Ti, Zn, Fe, Zr, Ce, Bi, Li, Na, K or protonated nitrogen base are known. For example, U.S. Pat. Nos. 5,780,534 and 6,013,707 disclose that calcium phosphinates and aluminum phosphinates of Formula (II) are said to be particularly effective in polyester.
As is common with many flame retardant systems, the performance of phosphorus containing acid derivatives can be enhanced by the presence of other flame retardant agents, synergists and adjuvants. U.S. Pat. No. 6,472,448 discloses flame retardant rigid polyurethane foam wherein a combination of oxalkylated alkylphosphonic acids and ammonium polyphosphate is present as flame retardant.
U.S. Pat. No. 6,365,071 discloses a synergistic flame retardant combination for thermoplastic polymers, comprising A) a phosphinic salt of the formula (II) above and B) a nitrogen compound such as allantoin, benzoguanamine, glycoluril, urea cyanurate, melamine cyanurate and melamine phosphate.
U.S. Pat. No. 6,255,371 discloses a flame retardant combination comprising, A) a phosphinate of formula (II) above and B) condensation or reaction products of melamine e.g., melamine polyphosphate, melam polyphosphate and melem polyphosphate. U.S. Pat. No. 6,547,992 discloses a flame retardant combination for thermoplastic polymers comprising phosphinates and small amounts of inorganic and/or mineral compounds which do not contain nitrogen.
The phosphinates cited, e.g., in U.S. Pat. Nos. 6,365,071 and 6,255,371, are said to be thermally stable, however, these materials are not necessarily suitable for use in all polymer systems and may create problems for processing under more demanding conditions or may lack the flame retardant effectiveness needed for certain polymers.
The metal salts of compounds according to formula (Ia), are also reported to be thermally stable, but this is of course a relative term. As disclosed in US 2007/0029532, decomposition of such phosphonic acid salts is well known at temperatures encountered during processing of polyesters and polyamides, damaging the polymers in the process.
U.S. Pat. No. 5,053,148 discloses heat resistant foams obtained by heating metal phosphonates or metal phosphonate precursors to temperatures of above 200° C. useful, e.g., as electrical and/or heat insulation materials. Also disclosed is the use of this reaction to expand or render porous other substrates. Such substrates include, for example, thermoplastic polymers or plastics such as aromatic polyesters, polyethers, polysulfides, polyamides, polycarbonates, polyimides, polysiloxanes or polyphosphazenes, can be introduced into the foaming operation as a mixture with metal phosphonates and/or their precursors.
While U.S. Pat. No. 5,053,148 may suggest that a porous polyamide may be produced by heating a mixture of a metal phosphonate and a polyamide according to the “foaming process”, nothing in U.S. Pat. No. 5,053,148 addresses or refutes the disclosure of US 2007/0029532 that decomposition of such phosphonic acid salts at high temperature gives “brittle compositions which are unusable” as an engineering thermoplastic. Outside of suggesting that a porous foam may be produced by heating metal phosphonate and a polymer such as polyamide, U.S. Pat. No. 5,053,148 contains no mention of what the properties of such an unexemplified material might be.
The difficulty of thermally processing certain thermoplastic resins in the presence of alkylphosphonic acid metal salts, and the poor physical properties of the polymer composition obtained thereby, has been confirmed by experimentation.
Copending U.S. patent application Ser. Nos. 14/337,500 and 14/592,472 disclose that products obtained by heating certain alkylphosphonic acid metal salts, such as aluminum salts, calcium salts, zinc salts etc., at temperatures in excess of 200° C. are thermally stable at temperatures above 400° C. and can be thermally incorporated into many thermoplastic polymer resins without the adverse impact on physical properties of the polymer seen with many of the salts described above. However, certain polymers containing the flame retardants of Ser. Nos. 14/337,500 and 14/592,472 that are processed under extremely harsh conditions may benefit from further process stabilization.